Advantages of a permanent magnet electric machine include simple structure, reliable running, small size, lightness, little loss, high efficiency, and that its shape and dimension can be easily varied, so that it is applied to many fields which spread almost all over the daily life, agriculture, aviation, aerospace, national defense, and general industries.
Please refer to FIG. 1, which is a cross-section diagram showing a rotary structure of a conventional permanent magnet electric machine having an outer rotor. In FIG. 1, a permanent magnet motor having eight poles and six slots is used as an example to describe the rotary structure 801 of the permanent magnet electric machine. The rotary structure 801 includes a stator 30 and a rotor 40. The stator 30 on which a rotating magnetic field can be produced is cylindrical in shape and is fixed to the interior of the permanent magnet electric machine. The rotor 40 having a rotor magnetic field is annular in shape, coaxial with the stator 30, and around the stator 30. When the rotor magnetic field is interacted with the rotating magnetic field of the stator 30, a rotary motion is produced on the rotor 40.
The stator 30 of the rotary structure 801 includes a stator core 1, a stator shaft 2, and six windings 3. The stator core 1 is mounted on the stator shaft 2 and is composed of permeability magnetic material. Six salient teeth 5 wound by the six windings 3 are protruded from the stator core 1 for forming six winding slots 4 and six winding slit orifices 6. Driving currents flow through the six windings 3 for producing the rotating magnetic field of the stator 30.
The rotor 40 of the rotary structure 801 includes a rotor yoke 7 and eight permanent magnets 8. The rotor yoke 7 is annular in shape. The eight permanent magnets are peripherally spaced with equal intervals along a peripheral direction on an inner surface of the rotor yoke 7 and alternatively change magnetic polarities between N pole and S pole thereof. Each permanent magnet 8 is a magnetic pole composed of permanent magnetic material. The rotor 40 rotates around the stator 30 with respect to the stator shaft 2, and the space enclosed by inner surfaces of the permanent magnets 8 and outer surfaces of the salient teeth 5 and the winding slit orifices 6 of the stator 30 forms an air gap 9.
In FIG. 1, electric currents high enough are set up in the windings to make the rotor 40 rotate as required. As the permanent magnets 8 are interacted with the winding slots 4 and the winding slit orifices 6, a cogging torque is produced, which denotes that an torque fluctuation is induced when a magnetomotive force distribution of the permanent magnets 8 is interacted with an air gap permeance distribution produced by the existence of the slots in the stator 30. Therefore, according to the definition, a torque induced by rotating of the rotor is the cogging torque in the absence of the driving currents.
The problem caused by the cogging torque is that the cogging torque results in the fluctuation of an output torque, influences the rotation thereof, and produces a speed fluctuation, vibration and noise.
At the same time, losses such as copper loss and core loss in the interior of the electric machine can make the iron core, the windings and the permanent magnets heated, thereby causing the temperature of the electric machine to rise. The air gap between the stator and the rotor in much electric machinery is used to form an air channel for air heat radiation.
The performance of the electric machine is significantly influenced by the configuration of the air gap between the stator and the rotor. In general, the air gap cannot be too thick so that the effect of air heat radiation is confined, thereby making the temperature of the electric machine too high.
Please refer to FIG. 2, which is a cross-section diagram showing a rotary structure of a conventional permanent magnet electric machine disclosed in the US Publication No. 2005/0258698 A1 and having a low cogging torque and a high torque density. In FIG. 2, P pairs of permanent magnets are peripherally spaced with equal intervals along a peripheral direction on a surface of a rotor iron 11 or a rotor shaft 11 of the permanent magnet electric machine. A magnetic pole surface 13b of each permanent magnet 13 has an arc surface 13c and a pair of inclined surfaces 13d. The arc surface 13c is in the middle of the magnetic pole surface 13b along the peripheral direction and forms an air gap with a stator of the electric machine. The pair of inclined surfaces 13d is located on two sides of the arc surface 13c along the peripheral direction and is inclined to deviate from the surface of the stator magnetic pole when being away from the arc surface 13c, so as to make the air gap gradually expanded.
Here, a first pole arc ratio Ψ1 for the arc surface 13c of the each permanent magnet 13 is defined as Ψ1=θ1/(180/2P), and a second pole arc ratio Ψ2 for the each permanent magnet 13 is defined as Ψ2=θ2/(180/2P). As a result, a non-dimension angle corresponding to the pair of inclined surfaces 13d along the peripheral direction satisfies a range of P/K≦Ψ2-Ψ1≦1.38×P/K, and an inclined angle θ for the pair of inclined surfaces 13d with respect to a radial plane PS of the each permanent magnet is given within a range of (70°-45°/P)˜(80°-45°/P), wherein K is the number of winding slots in the stator of the permanent magnet electric machine.
Although it can decrease the cogging torque that the pair of inclined surfaces 13d is added on the two sides of each permanent magnet 13 along a peripheral direction, the inclined angle given in the US Publication No. 2005/0258698 A1 for the pair of inclined surfaces 13d of each permanent magnet 13 cannot accomplish a very good effect to decrease a cogging torque, while applied to an electric machine having an outer rotor. Additionally, the inclined angle given in the US Publication No. 2005/0258698 A1 for the pair of inclined surfaces 13d of each permanent magnet 13 only considers the cogging torque and the torque density but doesn't consider air heat radiation and a windage loss of the rotor for the electric machine, wherein the windage loss denotes a mechanical loss produced by the inter-rubs of the rotor surface rotating at high speed with air.
In sum, in order to better decrease a cogging torque for a permanent magnet electric machine having an outer rotor, an inclined angle of a pair of inclined surfaces for each permanent magnet should be further determined in accordance with experimental results considering air heat radiation, and a windage loss of the rotor for the electric machine.